P A S T
30 000 BC : Proxima Centauri becomes the closest star to the Sun and succeeding the close binary and variable star(s) Gliese 65 AB or more commonly named the flare star UV Ceti, that today is in the constellation of Cetus. (Spectral Type M5.5). Proxima remains the closest star for the next 65 000 years. In 2005 AD) the distance is given as 2.683 parsecs (8.75 light-years) having visual magnitudes of 12.5 and 13.0, respectively.
2 300 BC : α Centauri is twice the apparent distance away from β Centauri as seen today. The Pointers, as such, do not exist in the ancient literature
1 000 BC : Magnitude of α Centauri is exactly 0.00.
185 A.D., 7th Dec. : α Centauri becomes rivalled by a ‘new star’ observed by the Chinese and remained visible to the naked eye for eight months before fading from view. Analysis by Hsi Tze-tsung (1955) ties down the date and likely position (l=282o and b=0o) though Lundmark in 1921 gives it high marks for being a true and reliable event. Clarke and Stephenson in the “Historical Supernova” (1977) conclude this was a supernova event. They also conclude the object may correspond to the infrared object RCW 86 (G315.4-2.3), which lies about half-way between the open cluster NGC 5866 and α Cen, or alternatively 2.3′S of α Cen itself.
1689 AD, December : Father Richard, a Jesuit priest, discovers the duplicity of α Cen from Ponicherry in Southern India.
1752 AD : First meridian circle observations by explorer Lacaillé. He gives rough estimates of the pair's positions. It is clearly obvious that the system is in real connection by comparing Richard’s and other early telescopic observations. This is not realised until Sir William Herschel in 1801 states the possibility that stars can exists as binaries.
1826 : James Dunlop measures the stars as ΔRA 1.783 and ΔDec. as 18.788 (22.45″), translating into the position angle as 56o 49′ (212.66o). He also gives the magnitudes as 1st and 4th, and oddly describes the colours of the two stars as ‘strong reddish yellow’.
1831 to 1834 : Thomas Henderson from the Cape of Good Hope makes the first parallax measurements using a mural circle finding an initial distance of 4.21y. These results however are not published due to doubt in the correctness of his measurements.
1834 : Sir John Herschel makes the first reliable micrometric observations of the pair and till 1996.417 others are made by other observers. He constructs an open-air observatory in Claremont, some six miles west of Capetown, installing a 0.45m (18-inch) 20-foot reflector. Due to the apparent changes since the observations by Lacaillé and others. Herschel starts conducting more frequent observations between 1834 and the beginning of 1838, hopefully to ascertain the orbital parameters and to improve the parallax measurements.
1838, December : Frederick Bessel publishes some parallax measurements prior to Henderson, even though Bessel himself did not make the first measurement.
1839, February : Henderson’s results published.
1848 : Discovery that α Centauri it is the closest of all the bright stars.
1870, 26th Sept : Assistant Government Astronomer H.C. Russell produces the first measurements from the 18.4cm (7.25-inch) refractor. Position Angle 21.0o, Separation 9.8″. α Cen is measured by him thirty-six times, on one complete side of the orbital ellipse in the next twenty years, contributing significantly to the precision of the orbit we see today.
1874, 13th June at 6.00pm: ‘First-Light’ for the 29cm (11.5-inch) refractor at Sydney Observatory. Russell measures PA as 30o 02′ The average separation found to be 7.71″. Seeing for the night described as excellent. Comment on the difference between the two refractors:
“The small star looks a darker yellow than the larger one.“ (Figure x.)1874-1894 : Henry Chamberlain Russell makes further observations from Sydney Observatory. Information about the binary nature of the system published in the Sydney Morning Herald in 1887.
1891 : Final agreement is reached on the system’s true attachment and period.
1904 : Thomas Wright measures the radial velocity of the system using a spectrograph. α Cen A has the measured velocity of -19.1 kms.-1, with the α Cen B measuring -24.47 kms.-1. The mean velocity of the system is calculated as -21.7 kms.-1 (towards us).
1915 : R.T.A. Innes discovers Proxima Centauri during his proper motion surveys from South Africa by blinking microscopy. It proves to be the closest star because of its position in the current orbit with the A+B system. Proxima is thought to be associated with A+B, even though it is 2.2o south-east. In the future, the proper motion will slow down because of the true connection with A+B.
1926 : Roberts, See, Doberek and Lahse calculate the first accurate orbit for the system.
1930 : The system is found to be approaching the Sun by the observer Schlesinger at a mean value of-22.2 kms.-1. This is 0.5 kms.-1 lower than the current accepted value.
1933 : A distance of 4.2 light-years is universally adopted.
1948 : A distance of 4.3 fight-years is universally adopted.
1955 : Closest approach in the apparent orbit by the separation of 1.61″.
1955, August : Closest approach in the true orbit (periastron) at 11.9 A.U.
1963 : My first observation through a telescope.
1967 : Thackeray determines the radial velocity of Proxima is -15.7±3.3 kms.-1.
1971 : First observation with my own telescope.
1978 : Kamper and Wesselink calculate that the total mass of all three stars is 2.13
Solar Masses. They also find the distance of Proxima to be 1.295 parsecs (4.22 ly.), and the AB system as 1.333 parsecs (4.35 ly.) but 4.3 light years remains the adopted distance.
1978 : Flannery and Ayres determine the age of the system, based on the luminosities and masses, as slightly older than the Sun at 6 billion years.
1980, May : Maximum apparent separation of the system at 22.6″.
1981, January : The ‘A’ star is found to be slightly variable, at around 0.65 magnitudes over a period of twenty minutes.
1986: Van Altena at Yale University calculates the age of the system as 5.5±0.2 billion years.
1993 : Matthews and Gilmore question the association of Proxima Centauri to the AB system. The velocity of Proxima as measured by Thackeray is too high for physical connection. They conclude that Proxima is most likely a star passing close to the AB system. Later, using the ESO’s Coravel program in this same year, the results based on probability made again by Matthews and Gilmore reverse this decision, concluding a likely association.
1994, January : Time of apastron in the true orbit. Orbit begins to close.
1997, August : Hipparcos parallax data is finally released. Adopted distance is now 1.3478±0.0026pc. or 4.3955±0.0082 ly. from the most accurate parallax known to date of 0.74212″±0.00140 arcsec. This distance is universally adopted, but in reality decreases measurably from year to year.
1999, May : Begins more notable differences in the telescope.
2000 : Position Angle starts changing more rapidly until around 2025. Apparent visual magnitude (v) is -0.29.
2002 : A new paper published in Astronomy and Astrophysics of another set of orbital elements with higher precision. This was made by a collaboration whose author name listed as the Belgian observer D. Pourbaix. This is abbreviated in the US Naval Observatory (USNO) WDS Reference file as Pbx2000b. One of the main differences is the quality grade of the orbit, which is downgraded to 2.
F U T U R E
2005-2009 : The Position Angle and separation continue to change with the orientation is now notable different. Using the orbital element of Pbx 2002, the following position angles and separations are calculated as follows;
****************** YEAR PA. Sep. ****************** 2005 229.7 10.574 2006 231.8 9.814 2007 234.3 9.050 2008 237.3 8.287 2009 240.9 7.532 ******************2013, June : Position Angle is exactly 270o
2016, February : Separation closes to minimum of 4.01″. on western or following side. (PA=300.4o)
2020, July : Position Angle is exactly 0o north.
2029, September : Secondary maximum separation reaches 10.44″. (PA= 19.8o)
2035, May : Periastron of true orbit at 35 AU. Thi is the first since 1955.
2037, August : Position Angle is exactly 90o at 1.85″.
2037, November : Separation closes to 1.71″ eastern or preceding side. (PA is 112o) Changes in Position Angle for five or six months reaches 5o degrees each month.
2038, May : Closest approach in Apparent orbit at 2.58″.
2060, May : Maximum apparent separation again 22.6″.
2073, November : The system in the true orbit again reaches apastron.
2 998 A.D : The AB pair of α Centauri crosses the galactic plane at longitude ‘L’ 314.95o.
4 000 AD : α Centauri and β Centauri point to the true centre of the Cross.
6 200 AD : α Centauri rapid common proper motion produces a close approach or stellar conjunction with β Centauri. The minimum distance reaches 23′ or 1380″. This is the best stellar conjunction for 1st magnitude stars for the next four hundred thousand years.
10 000 AD: Barnard’s Star makes its closest approach to the Sun at 1.153 parsecs or 3.76 light years. This is about 0.04 parsecs further than α Centauri would be at this time. Barnard’s star reaches a maximum visual magnitude of 8.5, one magnitude brighter than the current 9.5 magnitude. Currently this star is in Ophiuchus, near 66 Oph (V2048 Oph). Sky Atlas 2000.0 has an insert showing the position between 1900 and 2100 AD. By 10 000 AD it will be a far southern object.
11 000 - 13 000 AD : The true orbit closes until parallel to the Earth’s orbital plane. The orbit changes from direct motion to a retrograde motion. The system acting like an eclipsing binary presently appears unlikely, as system period is too long and will likely miss.
17 400 AD : α Cen passes south of Gacrux (γ Crucis) by 34′. (1440″)
27 000 AD : α Cen lies at exactly 1 parsec away (3.26 light years) from the Sun.
29 240±1 370 AD : α Cen reaches its closest approach to the Sun at a mere 2.970±0.012 light years. The measured parallax reaches 1.098″. Apparent magnitude reaches -1.05±0.08, just greater than the current brightness of Canopus.
31 000 AD : The system has moved 45o in the sky, approaching the border of the equatorial constellation of Hydra.
35 000 AD : The red dwarf star Ross 248 in the constellation of Andromeda (0.3oSE of Chi (χ) Andromedae) nearly becomes the closest star to the Sun at the distance of 3.023 light years or 0.927 parsees. This has a calculated error, according to Matthews (Aug 1993) of +2 300 years. Today (2005), Ross 248 has the higher radial velocity of -79.2±0.9kms-1. By this date, the velocity has increased by another 3.8kms.-1. Visual magnitude has changed from today’s 12.3 to 9.6 magnitude.
40 000 AD : Alpha Centauri becomes a northern hemisphere object.
41 000 AD : The star AC+79o3888 in the present constellation of Draco now (2004) some 3.4oN of Lambda (λ) Draconis then takes over from Ross 248 as the closest star at a distance of about one parsec.
43 000 AD : AC+79o3888 reaches its nearest distance of 0.971 parsecs or 3.17 light years. The maximum magnitude for this red dwarf star reaches 7.2.
46 000 AD : Ross 248 becomes again further from the Sun than Proxima. However, Proxima in its long 100 000 year orbit becomes further than the A+B system.
50 000 AD : α Cen becomes again the closest star.
56 000 AD : α Cen again lies 4.4 light years away.
c. 2 000 000 AD : 2 million A.D.! : The perturbations by the close approach of the α Cen system, so long ago, causes comets from the Oort Cloud to ‘rain’ down towards the Sun. According to the calculations of Bailey and Stagg (1990) and by Matthews (1993), over a period of twenty thousand years, a maximum of two hundred thousand comets may cross Earth’s orbit. equal to an unbelievable ten naked-eye comets per year. For the Earth’s population, fear from a planetary collision would probably be realistic, though for the comet hunters - this would likely be pure heaven! Although they might end up being “snowed under”!)
Note 1: The gravitational effects on the planets would not be adverse to their current orbits. as the effects are thousands of times too small.)
Note 2 : Distances are quoted from calculations made by Jahreiss and Morrison in 1993 using the Gliese Catalogue. (The Gliese Catalogue is based on the closest stars to the Sun.)
In finding and calculating some of the information in the history above, Robert Matthews’ “The Close Approach of Stars to the Solar Neighbourhood” by (QJRAS, 35, 2, p.1-9 (1994)) has proved invaluable in calculating the changing positions and brightness of α Cen. This short and informative paper discusses the approach of other stars to the Solar System in the next 100,000 years, and also provides the formulae to do so. Some values quoted in this text have been updated to include information from the Hipparcos satellite data.
Data on the measurements featuring in Figures 2 & 3 on α Centauri’s orbit (published with Part 1 in the July issue) were kindly obtained from Dr. Geoff Douglass of the US. Naval Observatory.
Approximate apparent separation between any two stars in space is obtained from the parallax and the angle between them. This is easily calculated by:
Where:
TS1= True Separation expressed in parsecs or light years.
s= Separation in arc seconds.
D= Distance expressed in parsecs or light years.
Or, if distance is required in either kilometres or Astronomical Units then:
Where;
p = parallax in arc seconds.
To calculate the estimated period in solar years (which will be fairly inaccurate), apply Kepler*s Second Law using distances in astronomical units:
(NO ADS!)